The present invention relates in general to wireless communication systems and in particular to an efficient method of and apparatus for improving the implementation of fast forward power control during soft handoff.
In a CDMA (code division multiple access) cellular system complying with an industry standard specification as set forth in IS-95A, the repetition rate of the forward link power control signals is slow, on the order of 50 Hz. The MS (mobile station) reports the status of the forward link frames to the SBS (selector bank subsystem) of a BSC (base station controller) via the entire set of BTSs (base station transceiver subsystems) in soft handoff with the MS. Based on the frame quality, the power control algorithm, as implemented by the SBS, determines the new transmit power levels for all the BTSs in soft handoff with the MS. The SBS relays the new transmit power levels to the BTSs. Hence, the power level used by each BTS in soft handoff with the MS is always equal. That is, the forward link power allocated to the MS at each of the BTSs is always synchronized to the same power level.
When a forward link frame is transmitted to the MS from the BTSS, it takes a finite amount of time before the MS receives it. If a communication mode designated as Rate Set 2 is employed, then the MS encodes an EIB (Erasure Indicator Bit) into the next outgoing reverse link frame, signifying the status of the forward link frame. After additional propagation delays, each BTS in soft handoff with the MS relays the demodulated reverse link frame and associated frame quality metrics to the SBS. The SBS then updates the forward transmit power levels of the forward link based on the EIB and instructs the BTSs to use the new level. Therefore, the time between the transmission of a forward link frame and the corresponding increase/decrease in transmit power is a constant delay. The delay is based upon the architecture (propagation/processing delays of the various interconnecting blocks) of the system. In rate set 1, on the other hand, the power control is accomplished through the use of the reverse link xe2x80x9cPower Measurement Report Messagexe2x80x9d (PMRM), which is triggered by a count of the number of bad forward link frames.
The fact that the power control process is slow implies that under certain channel conditions, specifically low speed movement of the MS and single multipath environments where long deep fades are expected, a high average forward transmit power is required to meet a given GOS (grade of service).
An evolution of CDMA, popularly designated as 3G (third generation), includes a fast forward link power control scheme wherein the MS determines whether or not it requires more forward link power to maintain the GOS. The decision is transmitted rapidly to the BTSs via a reverse link dedicated control channel.
With the introduction of fast forward link power control, there was the expectation that forward link capacity would increase by large amounts at low mobile velocities. However, all the published analyses, known to the present inventors, carried out to characterize the actual capacity gain have failed to consider the performance of the algorithm during soft handoff. The term xe2x80x9csoft handoffxe2x80x9d throughout the remainder of this document is intended to define the situation where an MS is in communication with two or more BTSs preparatory to the potential of being transferred from one cell to another. The independence of the reverse links during soft handoff, in terms of slow/fast fading and distance/antenna related path losses to the mobile, result in different raw bit error rates. Individual BTSs that are in soft handoff, rapidly control their transmit power based on the forward power control bits they demodulate from the reverse link. This may, and often does, result in a deviation in instantaneous transmit power at the different BTSs. Depending on the degree of soft handoff, the average path loss difference between mobile and BTSs, and the power control parameters used, the resulting required forward link transmit power for a user may be quite high due to this mismatch. This translates to capacity degradation.
The main conclusion that may be drawn from the above, is that, to understand the practical effects of fast forward power control on capacity, one must consider the fact that an MS may be in a soft handoff mode for a significant amount of time. During this time, there is substantial opportunity for the transmission power levels of the different BTSs in soft handoff with the MS to become substantially non-synchronized. Additionally, one or more soft handoff forward links may be inadequately controlled due to poor reverse links. Such a situation detrimentally affects the potential capacity of such a system.
A CDMA system using fast forward link power control is more fully disclosed and discussed in one of several co-pending patent applications, such as xe2x80x9cFAST FORWARD LINK POWER CONTROL IN A CODE DIVISION MULTIPLE ACCESS SYSTEM,xe2x80x9d filed Sept. 17, 1997, having application Ser. No. 08/932,093, to Chheda et al, and assigned to the same assignee as the present invention. This application is hereby incorporated into this document in its entirety by reference.
To realize the fast forward power control technique, the MS estimates the SNR (signal to noise ratio) per power control group after maximally ratio combining the received signal energy per multipath. The estimation is compared to a threshold and, based on the comparison, a power control, up or down, command is generated. The MS adjusts this SNR threshold every frame, or at 50 Hz, based on the quality of forward link frames. Each time a forward link frame is received in error, the threshold is increased. This represents situations where the SNR may be insufficient for the current mobility conditions. If the forward link frame is good, the SNR threshold is reduced. This is based on the assumption that the SNR is sufficient for the given GOS levels. The increase and decrease of the threshold are related by a FER (frame error rate) requirement. The up/down decisions are the control mechanism that ensures that the received SNR or the forward link is equal to the threshold.
During soft handoff, the different BTSs, of a 3G fast power control system, must now independently demodulate the MS power control decisions. Based on the demodulated decision, the BTS will increase/decrease its transmitted power. It is quite likely that the reverse links between the MS and the different BTSs will have different instantaneous power control bit error rates. Consequently, the actual decision that each BTS makes may not be the same. In other words, some BTSs may demodulate the power control bit decision in error. With the existing prior art network architecture, the BSC cannot synchronize the BTSs to the same power level after each power control decision because of the inherent processing and queuing delay of the central processing unit and interconnecting communication links.
If the reverse links from MS to BTSs were error free, then the transmitted power at each BTS per power control group time segment would be equal and perfectly synchronized. However, as mentioned above, each BTS to MS reverse link is likely to have a different instantaneous error rate due to the independent slow/fast fading, and different distance/antenna related path losses. Thus, if each link has a different error rate, then the actual transmitted powers from each BTS to MS during soft handoff deviate. This may result in loss of diversity. This is further explained via a possible scenario discussed below.
It may be assumed that the MS is in two-way soft handoff with the network. If the BTSs could be perfectly synchronized, then they would power up/down together. Hence, the diversity gain is optimized; when one path fades, the other is used and the reverse scenario also holds. In practice, even when the BTSs start out at the same level transmitted power level, due to different power control bit error rates per reverse link, their transmit power levels and up/down decisions are no longer perfectly synchronized. Typically, one reverse link will have a much higher FER and BER (raw bit error rate) than the other reverse link. If the MS requires lower forward power, then one BTS will power down quickly and appropriately, while the other may well remain at the same power level (if the raw BER is 50%), or power down at a much slower rate. Stability is usually maintained in such cases. Stability, for the purposes of this document, means that the FER requirements are maintained and the call drop rates are not increased. Since one BTS is not powering down, the other will power down to a lower level than it would if both links were adequately controlled. This ensures that just enough signal energy is provided to maintain GOS.
However, the converse is equally probable. Once the forward link is insufficient to a meet a given GOS, only the BTS with the more accurate reverse link will be able to react faster and more accurately by increasing its transmit power. Therefore, the resulting BTSs power levels are likely to be significantly different. The diversity gain is affected as a function of the difference between these levels. In other words, the forward link associated with the weaker reverse link is unable to provide diversity gain due to its inability to increase its transmit power. Thus, the overall requirement on power is higher in order for the adequately power controlled link to make up for the loss of diversity. Even if the weaker reverse link improves, the BTSs remain out of synch by an indeterminate factor as they adjust their powers up or down. If the other reverse link corrupts, the mismatch may increase or narrow.
It is important to keep in mind that while the higher reverse bit error rate on one link causes the traffic channel transmit power from one BTS to be much lower, the MAI (multiple access interference) that the BTS contributes at the MS receiver may still be high since, for the purposes of this discussion, it is assumed that the MS is in a soft handoff region. Thus the benefits of diversity cannot be realized. The overall effect is a reduction in system capacity. In fact, as the order of soft handoff goes up, this effect is more noticeable. Therefore, the soft handoff gain/benefit is compromised.
It may be noted that even if the MS were in a high order soft handoff and each link had independent 5% error rates, the traffic performance would still remain stable. This is because the forward powers of each BTS would not gravitate far apart. The problem is due to the fact that the error rates are xe2x80x9cburstyxe2x80x9d. By the term xe2x80x9cburstyxe2x80x9d, we mean that there are spurts of reverse link power control bit errors. Even though the overall reverse power control bit error rates are at 5%, one reverse link rate may have a power control bit error rate as low as a fraction of a percent, but occasionally go through error rate periods on the order of 20% to 30%. This causes the forward link power levels to significantly drift apart at each BTS, thereby resulting in a loss of diversity.
Finally, the overall power control coupling between forward and reverse link needs to be considered. If the MS is in a higher order soft handoff, and there is a poor forward link quality from a number of BTSs, the MS is more likely to make wrong reverse link power control decisions, and its general trend is to power down (lower the output transmitted power). This will in turn affect the power control bit rate and forward link frame quality, which further degrades the reverse link power control process. Eventually all forward link power control bits carried on the all the reverse links may thus be in error.
It would thus be desirable to find a way to improve the reverse link reliability during soft handoff, to ensure that such situations are minimized. It should be noted that there are practical issues, concerning present hardware and network limitations and standards, with synchronizing fast, that make rapid BTS power level synchronization a non-viable option at this time due to present day technology limitations and industry accepted standards. It is thus believed that a methodology is needed that synchronizes slowly and at regular intervals. Consequently, appropriate power control parameter selection, as a function of the state of soft handoff, is believed to be a prime factor in preventing capacity degradation.
The present invention comprises a method of improving the relative power level synchronization of all the BTSs communicating with an MS in a handoff mode through the use of at least one of 1) synchronizing all the handoff BTSs to the power level of the BTS having the best measured Eb/No (bit energy to noise density) of reverse link transmissions, 2) adjusting at least one of lower and upper limits of traffic channel gain of a BTS as a function of handoff mode and 3) adjusting at least one of the incremental FPC (forward power control) and RPC (reverse power control) parameters as a function of the number of BTSs in a soft handoff mode.